Method and device for monitoring a sensor

ABSTRACT

A method and a device are described for monitoring a sensor, in particular a sensor for measuring a pressure variable which characterizes the pressure of the air supplied to the internal combustion engine. A fault is detected if a change in an operating variable that characterizes the quantity of fuel to be injected results in no corresponding change in the output variable of the sensor.

BACKGROUND INFORMATION

[0001] The present invention relates to a method and a device for monitoring a sensor in accordance with the preambles of the independent claims.

[0002] A method and a device for monitoring a sensor that measures a pressure variable that corresponds to the pressure of the air entering the internal combustion engine is known from German Patent 40 32 451 A1. In that publication, various methods and modes of operation for monitoring a boost pressure sensor are described. The output signal of the boost pressure sensor is compared to a reference variable. Used as the reference variable is the output signal of a second boost pressure sensor. In a further embodiment, in certain operating states, for example, in response to small loads and rotational speeds, the output signal of the boost pressure sensor is compared to that of an atmospheric pressure sensor. In stationary operating states, the measured value is compared to a value that is calculated on the basis of the fuel quantity and the engine rotational speed.

[0003] Disadvantageous in this mode of operation is the fact that a further sensor is required or the monitoring can only be carried out in certain operating states. If these operating states are only seldom achieved, then under certain circumstances long operating phases result in which a defective boost pressure sensor is used.

ADVANTAGES OF INVENTION

[0004] Because a fault is detected if a change in an operating variable characterizing the quantity of fuel injected does not result in a corresponding change in the output variable of the sensor, reliable detection is possible, especially in the dynamic operation of the internal combustion engine. As the operating variable characterizing the quantity of fuel injected, various variables can be used. Thus it is possible to use different torque variables, such as the input torque, drive signals for quantity control elements, or other variables that arise in a controlling means for an internal combustion engine. It is particularly advantageous to use the mode of operation according to the present invention in a boost pressure sensor which measures the pressure of the air that is fed to the internal combustion engine. The mode of operation, however, can also be used in the case of other sensors.

[0005] Certain faults such as the icing up of a sensor only arise in response to specific operating states, so it is advantageous if the monitoring of the sensor is made to be a function of the presence of certain operating states.

[0006] The icing up of the sensor only occurs in certain ranges of the air temperature. The monitoring takes place if a variable that characterizes the air temperature is smaller than a threshold value. Monitoring is only reliable and/or capable of great precision if preestablished rotational speed values and/or fuel quantity values are present. The monitoring takes place if predetermined values exist for the rotational speed and/or the quantity of fuel injected.

[0007] It is particularly advantageous if a substitute value is used in response to a detected fault. One substitute value that is especially easy to calculate derives from the rotational speed and the quantity of fuel injected. By using the substitute value, it is possible to continue to operate the internal combustion engine even in the event of a fault, the control precision of the internal combustion engine being only slightly impaired.

[0008] Advantageous and beneficial embodiments and refinements of the present invention are characterized in the subclaims.

DRAWING

[0009] The present invention is discussed below on the basis of the embodiments depicted in the drawing. FIG. 1 depicts a block diagram of the system for measuring the boost pressure, FIG. 2 depicts a detailed representation of the monitoring of the boost pressure, and FIG. 3 depicts a block diagram for illustrating the calculation of a substitute value for the boost pressure.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0010] The mode of operation according to the present invention is described below on the basis of an example of a boost pressure sensor. However, the present invention is not limited to this application. The mode of operation according to the present invention can be used in all sensors in which a change in an operating variable results in a corresponding change in the output signal of the sensor. In particular, the mode of operation according to the present invention can also be used in a sensor for measuring the air quantity, a variable correlated with the boost pressure, or a variable characterizing the boost pressure. In particular, the mode of operation can also be used in a sensor for measuring the air quantity.

[0011] In FIG. 1, a sensor for measuring the boost pressure and the associated analog/digital converter are designated as 100. The latter supplies a signal UP, that corresponds to the boost pressure, to a characteristic curve 110. There, this variable is converted into a signal PR, which in turn is supplied to a filter 120. Output signal P of filter 120 is conveyed via a first switching unit 130 to a controlling means 140, which further processes this signal, in order, in this way, to drive the internal combustion engine or actuators that are arranged in the internal combustion engine.

[0012] Applied at the second input of first switching unit 130 is an output signal PS of a simulation unit 135. This simulation unit 135 calculates a simulated boost pressure PS on the basis of a variety of variables.

[0013] Switching unit 130 can be driven by a first monitoring unit 150. This means that in response to a detected fault, the first monitoring switches first switching unit 130 into a position such that output signal PS of simulation unit 135 arrives at controlling means 140. First monitoring unit 150 evaluates the signals of various sensors 160, which characterize, for example, fuel quantity injected QK and/or rotational speed N of the internal combustion engine. Furthermore, output signal PR of characteristics map 110 is advantageously evaluated for fault monitoring. Alternatively, or in addition, output signal P of filter 120 or output signal UP of the A/D converter of sensor 100 can be processed directly.

[0014] A further embodiment is depicted by the dotted lines. In this one, between first switching unit 130 and controlling means 140 a second switching unit 170 is arranged, which is driven by a second monitoring unit 180. In the event of a fault, second monitoring unit 180 drives switching unit 170 so that output signal PA of a delay unit 175 arrives at controlling means 140. This has the result that, in the event a defect is detected, the value most recently recognized as fault-free is kept.

[0015] The sensor output signal made available by an A/D converter is converted from characteristic curve 110 into a variable PR that corresponds to the pressure. After evaluating the various signals using the first monitoring and/or the second monitoring, various faults are detected.

[0016] As a result of the corresponding driving of first switching unit 130 and/or second switching unit 170, a substitute value PS, or a previously stored value PA, can be used by controlling means 140 as the substitute value in response to a detected fault, for controlling the internal combustion engine. For this purpose, delay unit 175 stores the value most recently recognized as fault-free. This old value PA, stored in delay unit 175, is then used in the controlling of the internal combustion engine.

[0017] Using the first monitoring and/or the second monitoring, various faults can be detected. Thus, for example, a signal-range check can be provided at a minimum and/or a maximum value for signal UP or signal PR. Furthermore, in certain operating conditions, a plausibility check can be carried out using a further sensor, such as an atmospheric pressure sensor.

[0018] In addition, it is possible in accordance with the present invention to provide that a plausibility check be carried out using the injection quantity and/or another operating variable that has an essential influence on the boost pressure. This plausibility check is advantageously carried out such that a fault is detected if a change in the operating variable does not result in a corresponding change in the output variable of the sensor.

[0019] As the operating variable, it is advantageous to use a variable which characterizes the quantity of fuel injected. For this purpose, on the one hand, it is possible to use a setpoint value for the quantity of fuel to be injected and/or an actuating variable that is used for driving an actuator that determines the fuel quantity. For example, the drive duration of an electromagnetic valve or of a piezo actuator is suitable. This monitoring is depicted in greater detail in FIG. 2.

[0020] If a corresponding fault is detected, then first switchover unit 130 switches over to simulated substitute signal PS. This means that the functional capacity of the sensor is monitored and, in response to a defect, substitute signal PS is used. For determining the substitute signal, variables are used which characterize the operating state of the internal combustion engine. The value determined in this manner is also filtered using a filter which has a delaying component. A more detailed representation of the derivation of the substitute value can be found in FIG. 3.

[0021] First monitoring unit 150 is depicted in greater detail, by way of example, in FIG. 2. In certain operating states, it can occur that boost pressure value UP remains constant although the actual boost pressure changes. A fault of this type is also termed a freezing or icing up of the sensor. To detect this fault, the fault monitoring depicted in FIG. 2 is carried out.

[0022] The monitoring according to the present invention only takes place in certain operating states. If an operating state of this type exists, in which the charge air temperature is below a threshold value TLS, and the rotational speed and the quantity of fuel to be injected are within certain value ranges, then after a reversal of sign in response to the change in the quantity of fuel to be injected, the instantaneous quantity and the instantaneous boost pressure are stored as old values QKA and PA, respectively. Simultaneously, a time counter starts up. After the conclusion of a waiting time, differences QKD between old stored value QKA and instantaneous value QK of the injection quantity are calculated. Accordingly, change PD in the pressure is determined in this waiting time.

[0023] If the amount of the difference between the fuel quantity values is greater than a threshold value QKDS, then the amount of the change in the boost pressure must be greater than a threshold value PDS. If this is not the case, then a fault is detected.

[0024] In FIG. 2, an embodiment of a monitoring device of this type is depicted by way of example. Supplied to a first comparator 200 is output signal TL of a temperature sensor 160 c, which makes available a signal that corresponds to the charge air temperature. Additionally supplied to comparator 200 from a threshold value input 205 is a threshold value TLS. Comparator 200 acts upon an AND gate 210 using a corresponding signal. Supplied to a second comparator 230 is the output signal of a characteristics map 220, at whose input rotational speed signal N from rotational speed sensor 160 a is applied. In addition, characteristics map 220 processes a variable QK, which characterizes the quantity of fuel to be injected and which is advantageously made available by a quantity controlling means 160 b. Also supplied to comparator 230 by a threshold value input 235 is a threshold value BPS. Comparator 230 also acts upon AND gate 210 using a corresponding signal.

[0025] Variable QK subsequently arrives at a sign detection unit 250 and a filter 260. Using the output signal of sign detection unit 250, a time counter 270 as well as a first storage unit 262 and a second storage 265 are acted upon.

[0026] The output signal of filter 260 arrives, on the one hand, directly bearing a positive sign at a node 285 and, on the other hand, via first storage unit 262 bearing a negative sign at the second input of node 285. Node 285 acts upon a switching unit 275 using a variable QKD. The output signal of switching unit 275 QKD arrives at a third comparator 280, at whose second input output signal QKDS of a threshold value input 285 is applied. Evaluation unit 240 is also acted upon by the output signal of comparator 280.

[0027] Output signal P of filter 120 arrives, on the one hand, directly carrying a positive sign at a node 287 and, on the other hand, via second storage 265 carrying a negative sign at the second input of node 287. Node 287 acts upon a switching unit 276 using a variable PD. The output signal of switching unit 276 PD arrives at a fourth comparator 290, at whose second input output signal PDS of a threshold value input 295 is supplied. Evaluation unit 240 is also acted upon by the output signal of comparator 290.

[0028] First comparator 200 compares measured charge air temperature TL to threshold value TLS. If measured charge air temperature TL is smaller than threshold value TLS, then a corresponding signal arrives at AND gate 210. Characteristics map 220, on the basis of at least the rotational speed and/or the quantity of fuel to be injected, calculates a characteristic value that characterizes the operating state of the internal combustion engine. This characteristic value in comparator 230 is compared to threshold value BTS. If the characteristic value for the operating state is larger than threshold value BPS, then a corresponding signal is sent to AND gate 210. If both conditions are fulfilled, i.e., the temperature of the air is smaller than the threshold value TLS and certain operating conditions exist, then a monitoring is possible.

[0029] This logic unit, composed of comparators 200 and 230, threshold value inputs 205 and 235, characteristics map 220, and the AND gate, results in the monitoring of the sensor signal as a function of the existence of certain operating states. The monitoring is carried out only if the air temperature is smaller than a threshold value and if certain values for the rotational speed and/or the quantity of fuel injected are present.

[0030] A check is carried out by sign detection unit 250 as to whether a reversal in the sign of the change of the fuel quantity is present. This means that a check is carried out as to whether the outlet has a zero throughput over the time of the fuel quantity to be injected. If this is the case, then in storage unit 262 the instantaneous values of the quantity of fuel to be injected are stored as old value QKA. Accordingly, in second storage 265, the instantaneous value of the pressure is stored as old value PA. In this context, it is particularly advantageous if the fuel quantity to be injected is filtered by filter 260 before being stored.

[0031] Simultaneous with the detected sign reversal, time counter 270 is activated. On the basis of instantaneous value QK and old value QKA for the fuel quantity, a differential value QKD is calculated in node 285, which indicates the change in the fuel quantity since the last sign reversal. Accordingly, in node 287, a corresponding differential value PD for the pressure is calculated, which characterizes the change in the boost pressure since the last sign reversal.

[0032] If the time counter has run down, i.e., a preestablished waiting time since the last sign reversal is elapsed, then differential signal QKD is compared to a threshold value QKDS using comparator 280. Accordingly, differential pressure PD is compared to a corresponding threshold value PDS in node 290. If the two values for the difference of fuel quantity QKD and differential pressure PD are each greater than the threshold value, then the device does not detect a fault. If only the difference in fuel quantity QKD is greater than the threshold value, and value PD for the pressure is smaller than threshold value PDS, then the device detects a fault. In this case, a corresponding signal for driving switchover unit 130 is produced by monitoring unit 150, i.e., by evaluation unit 240.

[0033] The mode of operation depicted here is one specific embodiment. Other specific embodiments are also possible, the check also taking place using other program steps. It is essential that a fault be detected if a change in an operating variable, such as the fuel quantity to be injected, results in no corresponding change in the boost pressure. If, after a sign reversal in the change in the fuel quantity, a change in the fuel quantity correlates with a change in the pressure variable, then no fault is present.

[0034] In place of the fuel quantity, other variables can also be used, which characterize the quantity of fuel to be injected, i.e., that are a function of the fuel quantity or that determine the fuel quantity. Thus, for example, a load variable, a torque variable, and/or a drive variable of a quantity actuator can be used.

[0035] In FIG. 3, simulation unit 135 is depicted in detailed form. The units already described in FIG. 1 are designated using the same signs. Signal N of rotational speed sensor 160 a and signal QK with respect to the fuel quantity to be injected arrive at a characteristics map 300, whose output variable is conveyed via a filter 310 to switching unit 130. Rotational speed N is also conveyed via a characteristic curve 320 and a node 330 to filter 310. At the second input of node 330, the output signal of a sign determination 340 is applied.

[0036] In characteristics map 300, a value is stored for boost pressure P as a function of the operating state of the internal combustion engine. This stored value corresponds to the boost pressure in the static state. In order to take into account dynamic states, filter means 310 is provided. This filter means 310 is preferably configured as a PT1 filter, and it simulates the temporal curve of the pressure in response to a change in the operating state. It is particularly advantageous if the response characteristic of this filter means can be varied as a function of the operating state of the internal combustion engine. For this purpose, in particular, characteristic curve 320 is provided, in which, as a function of at least rotational speed N, a variable is stored which determines the response characteristic of filter means 310, by weighting the time constants of the filter means.

[0037] In addition, the response characteristic of the sign determination is set, stipulating, as a function of the sign of the pressure change, a correction variable for correcting the output signal of characteristics map 320. The sign determination ascertains whether the pressure is rising or falling. As an input variable for the sign determination, it is preferable to use the output signal of characteristics map 300. It is particularly advantageous if two characteristic curves 320 are provided, the one or the other characteristic curve being used as a function of whether the pressure is rising or falling. 

What is claimed is:
 1. A method for monitoring a sensor, in particular a sensor for measuring a pressure variable that characterizes the pressure of the air supplied to the internal combustion engine, wherein a fault is detected if a change in an operating variable that characterizes the quantity of fuel injected does not result in any corresponding change in the output variable of the sensor.
 2. The method as recited in claim 1, wherein the monitoring of the sensor takes place as a function of the presence of preestablished operating states.
 3. The method as recited in one of the preceding claims, wherein the monitoring takes place if a variable which characterizes the air temperature is smaller than a threshold value.
 4. The method as recited in one of the preceding claims, wherein the monitoring takes place if preestablished values for the rotational speed and/or the quantity of fuel injected are present.
 5. The method as recited in one of the preceding claims, wherein, after a sign reversal of the change in the fuel quantity, a change in the fuel quantity is correlated with a change in the pressure variable.
 6. The method as recited in one of the preceding claims, wherein, a time period after the sign reversal of the change in the fuel quantity, the instantaneous value of the fuel quantity is compared to the value of the fuel quantity at the time point of the sign reversal.
 7. The method as recited in one of the preceding claims, wherein, in response to a detected fault, a substitute value is used for the controlling means.
 8. The method as recited in one of the preceding claims, wherein the substitute value is determined on the basis of the rotational speed and/or the quantity of fuel to be injected.
 9. A device for monitoring a sensor, in particular a sensor for measuring a pressure variable that characterizes the pressure of the air supplied to the internal combustion engine, wherein means are provided which detect a fault if a change in an operating variable that characterizes the quantity of fuel injected does not result in any corresponding change in the output variable of the sensor. 